The present invention relates to a thermal printer for printing image data on a paper or similar printing medium by thermal transfer. More particularly, the present invention is concerned with a thermal printer of the type applying hot melt ink, powdery toner or similar thermally meltable ink to an endless film belt being rotated, selectively melting the ink by a thermal head in response to a signal representative of image data, and transferring the melted ink to a recording medium to print the image data on the medium.
A thermal printer of the type described has a reservoir storing thermally meltable ink therein. The ink is liquefied by heat and supplied from the reservoir to the surface of a film belt via an inking member implemented as a drum. While the film belt is moved toward a thermal head, the ink deposited thereon is cooled and solidified. The thermal head melts the ink in response to an image signal sent from a control circuit, thereby transferring the ink from the film belt to a print paper or similar recording medium. This type of thermal printer has a problem in that while the transfer of the ink from the film belt to a print paper is repeated, the film belt is apt to skew due to the local error in the balance of the tension acting on the film belt. The local error is ascribable to various causes including the different degrees of parallelism of rollers constituting a belt drive mechanism, the different circumferential lengths of the rollers, and the slippage and mechanical stretch of the film belt.
To prevent the film belt from skewing, Japanese Utility Model Publication No. 19247/1990 discloses a device including a sensor responsive to the skew of the film belt, a solenoid operated by the output signal of the sensor, and a correction roller to be moved by the solenoid. However, this kind of implementation is not practicable without resorting not only to the sensor, solenoid and other mechanically movable parts but also to electric circuitry for driving them, complicating the construction and increasing the cost of the printer.